The present disclosure relates generally to the field of lithography, and more particularly, to a method and apparatus for reducing distortion to a mask during a pellicle to mask mounting process.
Optical projection lithography is a process by which an image on a mask is transferred to a substrate. A mask is typically included with a frame and a pellicle, collectively referred to as a mask system. The frame holds the mask and the pellicle in a transparent thin film that is mounted over the frame. The pellicle protects the mask from fallen particles and keeps the particles out of focus so that they do not produce an image, which may cause defects when the mask is being used. The pellicle is typically stretched and mounted over the frame and is attached to the frame by glue or other adhesive. The pellicle has become an indispensable component in optical projection lithography because of its defect prevention properties.
Existing pellicle application techniques use pressure to create an adhesive bond between the pellicle frame and the mask. During mounting, a pressure is applied to the pellicle in order to create a seal between the pellicle and the mask. The pressure distorts both the pellicle frame and the mask causing the flatness of the mask to degrade and cause registration errors on the wafer. Recently, experiments show that the stress due to pellicle mounting can contribute between 10 and 32 nm of inconsistent lateral pattern positioning at the mask. After a 4× reduction, the overlay accuracy has a pellicle-mounting component in the range of 2.5˜8 nm, which is undesirable. Therefore, what is needed is an apparatus and method to reduce pellicle induced distortion to the mask and overlay error.